Designers of automotive reciprocating internal combustion engines having understood for years that it is possible to control oxides of nitrogen through the use of exhaust gas recirculation (EGR). Most EGR systems have, however, been external. In other words, exhaust gas has been conducted from the exhaust manifold and into the engine through the intake manifold. Unfortunately, this scheme does not provide for very good control of EGR, particularly in engines in which higher manifold pressures are the rule, such as turbocharged and other types of boosted engines. In any event, high rates of EGR are difficult to manage even with naturally aspirated engines because combustion stability typically degrades rapidly with increasing EGR rate.
The present invention allows EGR to be managed internally through dual equal camshaft phase shifting while providing additional benefit of a variable duration exhaust event. The camshaft timing control allows heavy EGR, with concomitant benefits in terms of reduced oxides of nitrogen (NOx), and also decreased fuel consumption matched with excellent combustion stability. Dual or variable duration exhaust event provides an additional benefit inasmuch as the engine may be operated with retarded timing and a short exhaust event at light loads and low speeds, or with a short exhaust event and base timing at cold conditions and at low speed, wide open throttle operation. At high engine speeds, the camshaft may be operated at base timing with a longer duration exhaust event to provide increased engine power output. Additionally, the dual duration exhaust event allows better optimization of the timing of exhaust valve opening, which improves engine torque output at lower speeds.